This invention relates to a temperature sensor using a thermistor, and more particularly to a temperature sensor suitable for measuring high temperature such as the temperature of exhaust gases of an automotive engine.
Heretofore, a high temperature sensor has been known which carries out temperature measurements by taking out the variance in resistance value of the thermistor through respectively bringing the drawer type electrode into contact with the positive electrode surface and the negative electrode surface of the thermistor.
In the high temperature sensor of the type described, there has been a problem of increase in contact resistance between the thermistor and the drawer type electrode in the use for a long period of time. As a measure for solving this problem, a method is adoptable wherein platinum which is highly heat resistant and acid proof is coated in a state of paste and backed on the surfaces of the thermistor and of the draw type electrode thereof, for example. Such a method brings about a good effect to the thermistor. In other words, a high retention is obtained after the baking of the platinum paste. However, in the case of the shaft and the sheath that are acting as the drawer type electrode, the baking is made at the metallic surfaces, thus raising a problem in durability.
For example, a temperature sensor of a converter for exhaust gas treatment in an automobile is required to withstand a very high temperature such as 1,100.degree. C. Although the shaft and the sheath are made of materials highly heat resistant and acid proof such as a stainless steel or a nickel-chrome alloy "the commercial name-Inconel (phonetic)", oxidation still penetrates into the interior from the surface. According to results of experiments, the depth of penetration of said oxidation reaches 0.05 mm and sometimes 0.2 mm. Accordingly, it is necessary to fully withstand such a penetration of oxidation. It is conceivable to plate the shaft and the sheath with a precious metal for preventing oxidation, but this method is not satisfactory because problems such as pin-holes arise.
Further, in measuring the exhaust temperature of an automobile, it is necessary to fully withstand the vibrations imparted from the engine to the temperature sensor. Consequently, a mechanical strength as high as approx. 20 G is required.